Project Summary/Abstract The proposed study will test whether accelerated biological aging at midlife predicts higher levels of preclinical AD biomarkers, as measured with a highly reliable plasma beta-amyloid (A?) marker, and whether the A? accumulation contributes to cognitive decline in midlife. The causal basis of the associations will be established by studying 750 individual twins age 40-64 from the Louisville Twin Study (LTS), who were studied extensively in childhood. We will relocate and recruit the twins, conduct in person cognitive functioning and physical assessments, collect 50cc of blood to quantify biological age, plasma-A?, and genotype. Our ability to recruit twins to the new phase of study is demonstrated by our 2018 Midlife Pilot Study, in which we collected cognitive ability, physical measurements, and blood to construct a biological aging measure for 40 participants, with a consent rate of 100%. Preliminary results suggest that accelerated biological age is a better predictor of episodic memory than global functioning, consistent with prior research. The LTS is the only twin study capable of linking biological aging, A? accumulation, and cognitive functioning at midlife to cognitive functioning in childhood, as other comparable longitudinal twin study samples are currently too young. Our study will address two research questions that can only be answered by longitudinal twin studies with intensive childhood measurements: 1) Do people with genetic and environmental predispositions for accelerated biological aging have greater A? accumulation and perform worse cognitively in midlife?; and 2) Do early life developmental characteristics cause accelerated biological aging, A?, and midlife cognitive functioning? We will complete the following specific aims: Quantify several markers of biological aging to test whether accelerated biological aging predicts A? accumulation and lower cognitive functioning at midlife by collecting biological, cognitive, memory, and functional ability data from LTS twins age 40-64 (Aim 1); Determine whether A? accumulation mediates effects of accelerated biological aging on declining cognitive functioning, adjusting for confounding genetic and environmental factors (Aim 2); Evaluate effects of early life characteristics on accelerated biological aging, A? accumulation, and cognitive functioning in midlife, adjusting for confounding genetic and environmental factors (Aim 3). The significance of the proposed research is specifying causal effects of biological aging in midlife on A? accumulation and cognitive functioning, using blood-based aging measures to predict A? accumulation, and specifying causal effects of early life developmental mechanisms on accelerated biological aging, A? accumulation, and cognitive functioning at midlife. The innovation of the proposed work is the application of a longitudinal twin design, affording us the capability to establish causal effects of biological aging on preclinical AD outcomes in humans for whom experimental randomization is not possible; and establishing new standards for plasma-A? data collection in longitudinal twin studies. Consistent with NIA's aims, this project will test the clinical utility of biological aging measures as early indicators of ADRD risk.